Photodetector arrangements and circuits

ABSTRACT

An improved photodetector arrangement comprises positioning a photodetector having at least one semitransparent cathode so that the light beam striking the cathode passes through it at least twice before exiting from the photodetector. In another embodiment of the invention, a second photodetector is positioned relative to a first photodetector so that the light beam, after striking the first photodetector, strikes the second photodetector. Where the two photodetectors are employed, they may be electrically connected in parallel.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to improved photodetector arrangements andcircuits, and more particularly, photodetector arrangements and circuitsin which the light beam strikes or passes through one or morephotodetectors more than once.

In a number of analytical instruments, such as spectrophotometers, aphotodetector is employed to convert photon energy from a light beaminto electrical energy by photoelectric effect and the resultingelectrical energy is measured to determine certain characteristics ofthe light beam and/or characteristics of the material from which thelight beam has emanated before reaching the instrument.

The present invention is in improved arrangements of such photodetectorelements and their circuits to substantially improve the efficiency ofthe photodetectors and to increase the spectral response range of theanalytical instrument in which they may be used. In photodetectorarrangements and circuits incorporating the principles of the presentinvention, phototubes may be used which might otherwise have previouslybeen rejected for low energy response. Another advantage of thearrangements and circuits incorporating the principles of the presentinvention, is that the wavelength range of prior instruments may beextended to as much as 1000 nm without the addition of moving parts,without drastic modifications to preexisting equipment and withoutsubstantially increased expense. By extending the wavelength range ofsuch prior spectrophotometers, it is possible to analyze for phosphatesand other chemical compounds which are outside of the normal maximum 800nm wavelength range of prior spectrophotometers. Another advantage ofarrangements and circuits incorporating the principles of the presentinvention is the realization of increased signal-to-noise ratios of, forexample, in excess of 2,000:1 at 2A. Such improved signal-to-noiseratios maximize the quantum efficiency of the equipment and render theanalytical instruments useful in monitoring small changes at highabsorbence or trace components at low absorbence which were previouslydifficult or impossible to analyze.

To one embodiment of the present invention, an improved photodetectorarrangement comprises photodetector means having a cathode which is atleast semitransparent to a light beam and is capable of emittingelectrons when positioned in the light beam, wherein the cathode ispositioned relative to the light beam so that the light beam passesthrough the cathode at least twice.

In another embodiment of the present invention, return means ispositioned relative to the photodetector means for returning the lightbeam to the cathode after the light beam has passed through the cathodetwice so that the light beam strikes the cathode at least a third time.

In still another embodiment of the present invention, two photodetectormeans are positioned relative to each other so that the light beam,after striking the first photodetector, strikes the secondphotodetector.

And in still another embodiment of the present invention, at least twophotodetectors are electrically coupled in parallel to each other.

These and other objects, features and advantages of the presentinvention will be more clearly understood through a consideration of thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWING

In the course of this description, reference will frequently be made tothe attached drawing in which:

FIG. 1 is a schematic view of a phototube as conventionally positionedin the prior art so that the light beam strikes its cathode once;

FIG. 2 is a schematic view of a photodetector arrangement incorporatingthe principles of the present invention in which the photodetector hasbeen rotated so that the light beam passes through its cathode twice;

FIG. 3 is a schematic view of a second photodetector arrangement of thepresent invention in which a second photodetector is positioned toreceive the light beam exiting from a first photodetector;

FIG. 4 is a schematic view of still another embodiment of photodetectorarrangement of the present invention employing a pair of photodetectorsin which the light beam exiting the first photodetector is redirectedback through the photodetector prior to striking the secondphotodetector; and

FIG. 5 is a schematic view of a suitable electronic circuit whichincludes the two photodetectors as shown in either FIGS. 3 or 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A conventional photocell arrangement is shown in FIG. 1. In FIG. 1 aphototube 10 is shown having a semi-transparent cathode 12 and a platetype anode 14. The phototube 10 is shown as it has been conventionallypositioned in the past in analytical instruments, wherein a light beamLi directly strikes the cathode 12 once. Upon striking the cathode, thephoton energy in the light beam causes an electron emission from thecathode. The magnitude of this electron emission varies with thewavelength response of the particular cathode material and the amount ofenergy of that wavelength contained in the light beam to which thecathode is responsive. This electron emission may be read by suitableelectrical instruments from which the operator of the analyticalinstrument incorporating the photodetector may reach a conclusion as toone or more characteristics which are sought to be analyzed. Forexample, where the photodetector is in a spectrophotometer, the lightbeam reaching the photodetector will have emanated from a chemicalcomposition being analyzed. Depending upon the nature of the element orelements in the composition, various strengths of given wavelengths oflight will be present in the light beam Li striking the photodetector.Where the sample contains only trace quantities of elements having lowabsorbence qualities, or where the material is to be analyzed for smallchanges even at high absorbence, the photodetector must have a highenergy response and/or a high signal-to-noise ratio. If it does not,certain characteristics of the compound being analyzed may goundetected.

In the present invention it has been found that the emission generatedby a given phototube cathode may be substantially increased for a givenanalysis sample or a light beam by rotating the phototube as shown inFIG. 2 about 90° from that shown in FIG. 1. Referring to FIG. 2, whenthe phototube is rotated about 90°, the light beam will pass through thesemi-transparent cathode 12 twice, once upon entry into the phototubeand again upon exiting the phototube. It has been found that such doublepassage of the light beam through the cathode almost doubles thesensitivity of a given phototube as will be shown later in thedisclosure.

In FIG. 3 a second embodiment of photodetector arrangement incorporatingthe principles of the present invention is shown in which a secondphotodetector 20 is positioned to receive the light beam Le exiting fromthe first photodetector 10. In the embodiment as shown in FIG. 3, thefirst photodetector 10 is arranged in its rotated condition as shown inFIG. 2 so that the light beam passes through the cathode 12 of the firstphotodetector twice and upon exiting the first photodetector strikes thesecond photodetector 20.

In FIG. 4 a third embodiment of photodetector arrangement is shown whichalso employs two photodetector 10 and 20. This arrangement ofphotodetectors differs from that shown in FIG. 3 in that the light beamLe exiting from the first photodetector 10 after having twice passedthrough its cathode 12 is redirected by a suitable mirror or prismarrangement 22 back through the cathode 12 to again pass through thecathode before the light beam Le' passes on to the second photodetector20.

In the embodiments shown in FIGS. 3 and 4, it will be understood thatalthough one phototube 10 is shown in which the light beam passesthrough the cathode more than once, more than one rotated phototube maybe arranged in series to the light beam where the incoming light beam Liis of considerable strength. It will also be understood that the lastphototube, e.g. 20, in the embodiments shown in FIGS. 3 and 4 may beeither a vacuum photodiode or a solid state photodetector, such as asilicon photocell. In fact, such solid state photocell is desirableparticularly in the multiple photodetector arrangements shown in FIGS. 3and 4 as the last photocell because such solid state cells are extremelysensitive. This is beneficial where the light beam exiting from thefirst phototube or tubes is substantially weakened after passage throughthese preceding phototubes.

It will also be understood that although the mirror or prism arrangement22 is shown in combination with a second photodetector in FIG. 4, thatsuch mirror or prism arrangement may also be incorporated into thearrangement shown in FIG. 2 without the second photodetector.

In FIG. 5 a preferred schematic electrical circuit is shownincorporating the multiple photodetector embodiments shown in FIGS. 3and 4. The photodetectors 10 and 20 are arranged electrically inparallel to each other with the cathodes 14 of each of thephotodetectors being connected to ground through batteries 24 and 26,respectively. Where the two photodetectors 10 and 20 are connected inparallel, the first photodetector 10 is preferably continuously in thesystem and the second photodetector 20 may be selectively connected ordisconnected from the system by way of a switch 28 which may be operatedby a wavelength counter (not shown). Switch 28 is operative to connectthe second photodetector 20 either to ground or to its bias supply 30.

The outputs of anodes 14 of the photodetectors 10 and 20 are coupled viaconductor 32 to an amplifier 34 which includes a conventional 0% adjustand resistor R_(f) to control feedback.

Although not shown, it will be appreciated that a suitable electricalcircuit similar to that shown in FIG. 4 may be employed for the singlephototube embodiment shown in FIG. 2. In such circuit the electricalcircuit branch containing the second photodetector 20, its battery 26and switch 28 is eliminated.

Referring again to the arrangement shown in FIG. 2, it has been foundthat the efficiency of the semi-transparent photo cathode 12 isincreased by rotating the photodetector 90° throughout its spectrum byalmost a factor of two for the same wavelength and light level. By suchrotation, a simple single stage semi-transparent photodetector iseffectively operated as a two-stage photodetector without having to usethe high voltages normally associated with photomultipliers or multiplephotodetectors. Such increase in efficiency allows either the use of lowefficiency photodetectors which were not otherwise previously acceptablein many analytical instruments or, conversely, where the photodetectoris of high efficiency, the sensitivity of the instrument issubstantially increased. It has been found that rotation of thephotodetector as shown in FIG. 2 does not result in degradation of otherfactors such as photometric accuracy, dark current, drift or fatigue.

Where the second photodetector is employed as in the embodiments shownin FIGS. 3 and 4, not only is the quantum efficiency of the systemfurther increased, but also the cathode 12 of the second photodetectorcathode may differ from the cathode of the first photodetector toincrease the wavelength range of the analytical instrument. For example,the cathode of the first photodetector 10 may have an S-20 surface whichhas a response of from 190-800 nm or an S-5 surface having a response offrom 190-600 nm. On the other hand the cathode of the secondphotodetector 20 may have an S-1 surface having a response of from350-1000 nm or it can be a solid state photocell. Where the cathode ofthe first photodetector 10 is an S-20 surface and of the secondphotodetector 20 is an S-1 surface, the spectral range of the instrumentwill be increased to about 190-1000 nm. The switch 28 as shown in FIG. 5is particularly advantageous where the second photodetector has an S-1surface due to the relative noisiness of such surface.

By way of example, experimental data was obtained using a Perkin-ElmerModel 55, single beam UV-VISNIR spectrophotometer. A comparison of theoutput energy for both a single phototube positioned as shown in FIG. 1and a single phototube positioned in accordance with the invention asshown in FIG. 2 both of which have S-20 cathodes is shown in thefollowing table. In addition, the table also shows the output of aphotodetector arrangement as shown in FIG. 3 having first and secondphotocells having S-20 and S-1 cathodes, respectively.

    ______________________________________                                        Wavelength                                                                             Conventional                                                                              Rotated     Combination                                  in Beam  Arrangement Arrangement Arrangement                                  Li       (FIG. 1)    (FIG. 2)    (FIG. 3)                                     ______________________________________                                        190 nm    53 mV      104 mV      100 mV                                       200      158         319         294                                          300      247         450         420                                          325      204         380         356                                          ______________________________________                                    

The preceding four readings were with a deuterium lamp. The followingreadings were with a tungsten lamp.

    ______________________________________                                        300     55            104      101                                            325     166           325      315                                            350     325           661      630                                            400     1825          3170     3020                                           450     2030          2880     2800                                           500     1920          2580     2530                                           550     2150          2790     2770                                           600     2400          3090     3140                                           650     2450          3120     3200                                           700     2910          3570     3760                                           750     3390          4640     5000                                           800     750           1576     1870                                           825     430           1094     1370                                           850     230           763      1000                                           875     100           437      680                                            900     24            127      365                                            910     11            64       285                                            920     6             31       236                                            930     3.8           15       201                                            940     2.8           8.5      170                                            950     2.3           5.5      142                                            ______________________________________                                    

It will be understood that the embodiments of the present inventionwhich have been described are merely illustrative of a few of theapplications of the principles of the invention. Numerous modificationsmay be made by those skilled in the art without departing from the truespirit and scope of the invention.

What is claimed is:
 1. An improved photodetector arrangementcomprisingphotodetector means having a cathode which is at leastsemi-transparent to a light beam, said cathode having the capability ofemitting electrons when positioned in said light beam, said cathodebeing contoured and positioned relative to said light beam so that saidlight beam passes through said cathode at least twice while following asubstantially single linear path .Iadd.through and out of saidphotodetector means. .Iaddend.
 2. The arrangement of claim 1 includingreturn means for returning said light beam to said cathode after saidlight beam has passed through said cathode twice, so that said lightbeam strikes said cathode at least a third time.
 3. The arrangement ofclaim 1 wherein said photodetector means is a phototube.
 4. Thearrangement of claim 1 including second independent photodetector meanspositioned so that said light beam strikes said second photodetectormeans after said light beam has passed through the cathode of said firstphotodetector means at least twice.
 5. The arrangement of claim 4,wherein said second photodetector means is a solid state photodetector.6. The arrangement of claim 4 wherein said first and secondphotodetector means are responsive to light in differing wavelengthranges.
 7. The arrangement of claim 6 wherein said first photodetectormeans is responsive to light in the range of about 190-800 nm and saidsecond photodetector means is responsive to light in the range of about800-1000 nm.
 8. The arrangement of claim 4 wherein said first and secondphotodetector means are electrically coupled in parallel to each other.9. The arrangement of claim 8 including switch means for selectivelyuncoupling said second photodetector means from the parallel electricalcircuit with said first photodetector means.
 10. The arrangement ofclaim 1 further including second independent photodetector meanselectrically coupled to the first photodetector means in parallelrelationship.
 11. The arrangement of claim 10, wherein said firstphotodetector means is responsive to light in the range of from about190 nm to about 800 nm and said second photodetector means is responsiveto light in the range of from about 800 nm to about 1000 nm; andfurtherincluding switch means for selectively uncoupling said secondphotodetector means from the parallel electrical circuit with said firstphotodetector means.